Rotary drum dryers have been used in many environments for drying particulate materials. For example, in the asphalt industry, inclined rotary drum dryers are used for drying virgin aggregate before it is combined with recyclable asphalt or liquid asphalt, or both, to form an asphaltic pavement mix. In a typical drum dryer, there is provided a generally elongated cylindrical drum having a burner for flowing hot gases of combustion along the drum. Virgin aggregate is disposed in the drum for flow through the drum from one end to the other, either in a counterflow or parallel flow arrangement with the direction of flow of the hot gases of combustion. In a counterflow rotary drum dryer which is also used for mixing the dried aggregate with recycle asphaltic material or liquid asphalt, or both, the burner is typically disposed intermediate the ends of the inclined drum and aggregate is input to the upper end of the drum for flow in a drying zone toward the burner. Once past the burner, the dried aggregate is combined with the asphaltic materials in a mixing zone.
In drums of this type, flights are circumferentially spaced one from the other about and at various axial positions along the walls of the drum in the drying zone. Spiral inlet flights are provided to direct the virgin aggregate toward the drying flights in the drying zone. Once the aggregate has been dried, the aggregate is carried by combustion flights which preclude the aggregate from veiling in the flame of the burner. The aggregate then passes to the mixing zone.
The drying flights conventionally provide a veiling effect across the drum cross-section. That is, the flights will pick up moist aggregate adjacent the bottom of the drum and, as the drum rotates, distribute that aggregate in a veiling pattern across the drum. The aggregate veil efficiently absorbs heat from the hot gases of combustion flowing axially through the drum. It will be appreciated that there are significant differences in the drying characteristics of various aggregates. For example, it is not uncommon for aggregate of a certain size to veil only partially across the drum cross-section, leaving an opening in the veil through which hot gases of combustion may flow. That is, the hot gases will typically flow toward an area of least resistance and, hence, the exhaust temperature from the drum will rise if there is a significant opening through the veil. Various configurations of flights have been used to attempt to control the veiling action and, hence, adjust the exhaust gas stack temperature. For example, in U.S. Pat. No. 5,083,382, of common assignee herewith, adjustable flights with radially inwardly projecting dams are disclosed whereby the positions of the flights can be adjusted to alter the veiling effect. Those flights are adjusted by pivoting into selected angular positions dependent upon the type of aggregates used or other production parameters. The present invention affords an improvement upon such angularly adjustable flights by employing a plurality of identical flights which can be secured in different orientations to adjust the veiling pattern across the drum and, hence, adjust the exhaust temperature.
It will also be recognized that flights of different configurations have previously been used in drying drums to effect different veiling patterns. Hence, a supply of different types of flights was typically necessary for use with each drum, depending upon the desired veiling effect which, in turn, was dependent upon the type and gradation of the aggregate and other parameters of the drying process.